Urea assay

ABSTRACT

Urea assay or analysis process and reagents therefor, in which the assay involves a reaction between a sample of biological fluid and an acidic reagent solution of o-phthaldehyde and a chromogenic compound; the chromogenic compounds include certain substituted benzene compounds, certain substituted naphthalene compounds, certain aminopyrimidines, certain substituted quinolines, certain morpholino substituted quinolines and certain morpholino substituted naphthalenes.

This is a division of Ser. No. 740,612, filed Nov. 10, 1976, and nowU.S. Pat. No. 4,074,972.

I. INTRODUCTION

The present invention relates to the quantitative analysis of urea inbody fluids, such as blood plasma, serum, urine and cerebrospinalfluids.

II. VITAL SIGNIFICANCE OF ACCURATE AND RELIABLE UREA MEASUREMENTS

Urea is the major end product of protein catabolism in man, being theprimary vehicle for the removal of toxic quantities of ammonia from thesystem. Urea is a product which is principally formed in the liver andexcreted by the kidneys.

Thus measurements of urea in various body fluids provide the medicalclinician with a very valuable diagnostic indicator, e.g., as animportant measurement to assess kidney function or dysfunction andassociated disease states. Its measurement in serum is also used toprovide an accurate indication of proper levels of protein intake andconcommitant catabolism.

Generally, increased levels of urea are diagnostically significant.E.g., they are associated with nephritis, renal ischemia, urinary tractobstructions, and certain extrarenal disorders such as congestive heartfailure, certain liver diseases, and diabetes.

Decreased levels of urea likewise are diagnostically significant. Thatis, they may indicate acute hepatic insufficiency or may result fromovervigorous parenteral fluid therapy.

Changes, whether up or down, and even of a relatively small amount, areimportant. E.g., when following the progress of a particular diseasestate, it is imperative that accurate assays for urea be provided to themedical clinician so that early trends may be detected and treated,before worse and even potentially life-threatening conditions areallowed to occur. Thus accuracy should be sufficient that detection ismade of small percentage changes.

Therapeutic measures can be closer controlled by accurate determinationof changes, even changes of a small percentage increment.

Although the great importance of accurate and reliable urea assays hasbeen known for many years, the potential and future importance ofaccurate assessments of urea concentrations will likely continue to riseas newer drugs to combat a variety of human diseases are introduced tomedical practice; for experience has shown that many new drugs producetoxic side effects, and if such drugs are given in excess, they may leadto serious consequences to the patient's welfare as well as increasedhospitalization; thus excess dosage or the hypersensitivity of a patientto a particular drug may be established by accurate measurements ofurea, thereby preventing or minimizing adverse therapeutic reactions.

III. DISADVANTAGES OF PRIOR ART

An early urea measurement was that of Marshall..sup.[1] As early as1913, Marshall used the enzyme unrease as a tool for the determinationof urea in blood. The method consisted of incubation of the blood withurease, an enzyme which breaks down urea into one molecule of carbondioxide and two molecules of ammonia, isolation of the ammonia thusliberated by aeration, and quantitation of the ammonia by titration asan indication of the quantity of urea.

Nessler's reagent.sup.[2] was probably the most common reagent of thosewhich were used as an indication of ammonium ions (NH₄ ⁺) present, butNessler's method presented the burdensome restriction of photometricallyreading the color developed within a 1 minute time frame due to theformation of color by constituents other than NH₄ ⁺ ions reacting withthe reagent, leading to possible inaccuracy due to overestimation of theamount of urea.

Subsequent to the method of Marshall, a plethora of new methods havebeen suggested and attempted, due to the increasing awareness of thevital significance of accurate, reliable, and producible ureameasurements, and due to the fact that none overcame all disadvantageswithout introducing new ones.

These methods possessed the commonality of the enzyme urease, anddiffered only in the means of detection of the ammonia formed.

Perhaps the most widely used of the enzymatic methods was that whichemployed the Berthelot reaction.sup.[3] in which serum is treated withurease for about 15 minutes, and then two additional reagents are addedto the mixture and again incubated for about 4-10 minutes at an elevatedtemperature to produce the desired indicator reaction.

However, there continued to be disadvantages and difficulties with theentire class of methods which utilize an enzymatic breakdown of urea.Main disadvantages were the extended incubation times necessary to fullyreact the urea in the sample, and the stability difficulties of thereagent system; and perhaps the most serious disadvantage was the factthat these methods were not very well suited to the measurements ofurinary urea, for large amounts of free ammonia may be present in thesesamples and would be mistakenly measured as urea, which would lead tooverestimation and consequent erroneous diagnosis and treatment.

Even though the enzymatic methods were disadvantageous, the prior artmethods for many years continued to have that basic nature, even withthose disadvantages inherent in those methods; for the prior art haslong realized the benefits of urea assays, and even disadvantageous oneswere believed better than none at all, as the prior art continued to tryto achieve an advantageous urea assay.

In 1939, Fearon.sup.[4] departed from the urease enzyme methodologies,and showed that urea reacts with diacetyl monoxime at elevatedtemperatures in the presence of a strong acid and an oxidizing agent toproduce a chromogen. Ormsby.sup.[5] in 1942 applied the Fearon reactionto measure blood and urine urea in a protein-free solution. Althoughthese methodologies which employ the use of the Fearon reaction havegained rather wide acceptance today, especially in connection with theuse of automated chemical analyzers, they suffer one or more of thedisadvantages and drawbacks of: 1, the color developed isphotosensitive, thus requiring that the test be performed undercontrolled and minimal lighting conditions; 2, the lack of conformity toBeer's law, thus requiring the use of multiple standards; 3, theunpleasant nature of the reagents; 4, the fact that the reaction is notcompletely specific for urea, leading to inaccurate assays wheninterfering substances are present; and in many and perhaps even themajority of cases, the technician would not be aware of the presence ofthese substances, and thus would not suspect any cause for inaccuracy ofthe measurement; and, 5, requirements of rigid temperature control andthe use of very elevated reaction temperatures.

Although the Fearon method is perhaps the most widely used method today,the use of elevated reaction temperatures and the acidic nature of thereagents represents particular hazards when used in continuous flowanalysis equipment due to the buildup of pressure and the concommitantpossibility of disruption of plumbing, causing hot acid to be propelledinto the air and with serious consequences to the eyesight of laboratorypersonnel.

Subsequent to 1942, various investigators of the prior art have proposedstill other departures of the prior art have proposed still otherdepartures or approaches for the measurement of urea, including themanometric techniques (measurement of the pressure of gases released inthe reaction sequence). However, none have gained widespread acceptance,probably due to such disadvantages as the complexity of the method orthe requirement of expensive and troublesome equipment.

Thus, for many years, the prior art has struggled in various manners,and with differing approaches, in its attempts to discover a desirableurea measurement assay.

Other attempts by the prior art, in this long struggle for asatisfactory and successful urea determination, have long included anattempt to utilize the reaction between urea and an aldehyde to achievea colored reaction product or a reaction product which becomes coloredwhen reacted with a chromogen.

One of the earliest of these methods, attempting the use of an aldehyde,apparently was that of Brown, who attempted a urea determination by theuse of its reaction with p-dimethylaminobenzaldehyde (DMAB)..sup.[6]

However, despite the investigations of many others of the priorart.sup.[7], problems which were persistent with this approach of Brownwere the potential interferences with these methods by commonly useddrugs (with the attendant possibility of misdiagnosis) and thesensitivity of the final color developed to temperature fluctuation(necessitating expensive laboratory equipment to assure color stabilityduring the measurement of absorbance).

In 1973, still attempting a more desirable determination based on analdehyde reaction, Morin and Prox.sup.[8] presented a method based onthe reaction between urea and the aldehyde, p-dimethylaminobenzaldehyde(DMAB) in which they attempted to quantitate urea directly (without theneed for removal of the protein from the specimen) by measuring theabsorbance of the chromophore developed with the aldehyde. While theelimination of the necessity for protein removal was an improvement, themethod of Morin and Prox still suffered the problem of interference fromcommonly used drugs.

In 1975, Jung, et al..sup.[9] and Jung's U.S. Pat. No. (3,890,099)reacted urea with a different aldehyde, namely, o-phthalaldehyde, andthen went one step further and coupled the product of this reaction withN-(1-naphthyl)ethylenediamine dihydrochloride. Although Jung's approachhas certain advantages over other prior art, in that at least it doesnot require elevated temperatures for the development of thechromophore, it has drawbacks and disadvantages as now summarized.

First, the Jung method requires N-(1-naphthyl)ethylenediaminedihydrochloride. This is a material which is synthesized fromα-naphthylamine, and thus likely may contain least trace amounts ofα-naphthylamine, a compound which is widely known as a potent carcinogen(a cancer-causing agent)..sup.[10]

Further, as a possible hazard and disadvantage of the Jung method'srequirement of the N-(1-naphthyl)ethylenediamine dihydrochloride, thereis the disadvantage and danger of the unknown effect of the storage ofthat compound in the acid required in the reagent, which may decomposethe N-(1-naphthyl)ethylenediamine dihydrochloride, yielding theaforementioned carcinogenic α-naphthylamine.

Thus, there is the possible presence of α-naphthylamine in thelaboratory, with its attendant risks to the immediate and long termhealth of laboratory personnel.

Moreover, the Jung method shows significant interference from a class ofdrugs, specifically sulfa drugs, which are commonly used to treat thespecific disease states to which measurements of urea are used as acrucial diagnostic test; and thus the urea is mistakenly overestimatedto a certain extent in the present of those drugs, leading tounreliability and inaccuracy of the urea measurement.

The present invention is a distinct departure from the several prior artmethodologies, and it overcomes many of the disadvantages of prior artmethodologies. More specifically, in the assay of the present invention,the reagents are stable, the color reaction obeys Beer's law over a widerange of urea concentrations, does not require the use of elevatedreaction temperatures or uncommon laboratory apparatus, showssignificant immunity to interferences common to older methodologies,especially to drug-induced interference, and is extremely rapid(requiring less than 5 minutes to complete an analysis).

IV. THE PRESENT INVENTION AND ITS ADVANTAGES SUMMARIZED

This invention relates to a process for the quantitative measurement ofurea in aqueous or protein based samples. More particularly, theinvention relates to the discovery of major classes of chromogeniccompounds which will react with the condensation product of urea ando-phthalaldehyde, achieving more sensitive, specific, and therebyaccurate assays for urea, by producing a chromophore having verydesirable characteristics for clinical assays for urea.

The present invention overcomes the problems encountered in prior art inthat the chromophore is not photosensitive, and provides stable reagentsfor the assay.

A further inherent advantage of the present invention is that itmeasures urea and not free ammonium ion, thus making the methodadaptable to the measurement of urinary urea without costly andtime-consuming pre-treatment of the sample.

Further, in the present invention, the reaction obeys Beer's law (i.e.concentration versus absorbance readings are in direct linearproportion) over a wide range of urea concentrations, thus reducing thepossibility of error and allowing less occurrences of repeat analyses,thereby providing the clinician with more reliable results with aminimum of costly delay.

A further advantage of the present system over the Jung method is thefact that the present invention shows very little interference fromdrugs which are commonly used to treat the diseases for which themeasurement of urea is used to detect and monitor.

V. DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment of the present invention, a sample of a bodyfluid containing urea is added to a reaction tube; and to it are addedan acidic solution of o-phthalaldehyde, and an acidic solutioncontaining one of the general classes of chromogenic compounds listedbelow and given as examples. Chromophore development begins immediately;and the rate of color development may be hastened, if desired, byincubation of the reaction mixture at 37° C. Within a 3 to 5 minuteperiod, enough color is developed so that photometers commonly found inclinical laboratories may be used to measure the amount of color formed.The amount of urea present in the original sample is calculated bycomparing the absorbance of the patient's sample to the absorbance shownby a similarly treated standard solution of urea, the concentration ofwhich is accurately known.

The o-phthalaldehyde reagent is prepared by adding from 200 mg to 2000mg of o-phthalaldehyde to an aliquot of approximately 3.75 N sulfuricacid. To this mixture is desirably added a quantity of polyoxyethylenelauryl ether (such as BRIJ 35) or alkylaryl polyether (such as TRITON)or other nonionic surface active agents such that the finalconcentration of the surfactant is approximately 1-3% (w/v). The mixtureis then made up to a final volume of one liter.

This description will yield a reagent with a concentration of the majorreactive ingredient, o-phthalaldehyde, which will produce accurate andsensitive assays of urea in body fluids.

One may alter the concentration of o-phthalaldehyde within parametersgiven herein, depending upon the particular application of the reagentsystem. For instance, it has been found that increasing theconcentration of o-phthalaldehyde in this reagent will lead tosignificant rises in the rate of color development. Thus, for thelaboratory analyst who possesses high speed chemical analyzers, it wouldbe desirable to increase the concentration of the reagent so that thecolor may be developed and measured rapidly, thereby increasing theproductivity or throughput of the analysis. Conversely, if the procedureis to be run manually by a technician, it would be desirable to lowerthe concentration of this reagent in order to allow sufficient time forthe analysis to accomplish the steps necessary to process multiplesamples in an orderly fashion.

Thus it may be seen that the final concentration of the reagent employedwill vary depending upon the particular application involved, althoughall concentrations within the parameters given will lead to accurate andprecise assays.

It has been found that there are six major classes of compounds whichfunction as chromogenic compounds in this reaction. They are as follows:

(1) 1,3 or 1,3,5 di- or tri-substituted hydroxy or methoxy benzenecompounds, which have the following general structure: ##STR1## where R₁= --H or --CH₃ and where R₂ = --H, or --OH, or --OCH₃

Examples of this class are:

(a) 1,3-dihydroxybenzene

(b) 1,3,5-trihydroxybenzene

(c) 1,3-dimethoxybenzene

(2) 1, or 1,3 mono- or di-substituted hydroxy or methoxy naphthalenecompounds which have the following general formula: ##STR2## where R₁ =--H or --CH₃ and where R₂ = --H, or --OCH₃, or --OH.

Examples of this class of compounds are:

(a) (1,3-dihydroxynaphthalene)

(b) (1-hydroxynaphthalene)

(3) 4 or 4,6 substituted 2-aminopyrimidines, where the substitutinggroup is an electron withdrawing group possessing the following generalstructure: ##STR3## where R₁ is --H, or --OH, or --OCH₃ and where R₂ is--OH or --OCH₃

Examples of this class of compounds are:

(a) 4,6-dihydroxy-2-aminopyrimidine

(b) 4-methoxy-2-aminopyrimidine

(4) Those compounds which have the following general structure: ##STR4##where R₁ = --H or --CH₃ and where R₂ = --CH₃ or --C₂ H₅ or --H and

where R₃ and R₄ = --H or --CH₃ and

where n = 1, 2, or 3

An example of this general class of compounds is:

(a) 8-(4-amino-1-methylbutylamino)-6-methoxyquinoline

(5) Those compounds which have the following general structure: ##STR5##where R = --H, or --CH₃

R₁ = --ch₃, or --CH₂ CH₃ or --H or --CH₂ CH₂ --CH₃

X = o or C

n = 1, 2, or 3.

An example of this class is:

(a) 8-(2-N-morpholinoethylamino)-6-methoxyquinoline

(6) Those compounds which have the following general structure: ##STR6##where R₁ =--H, --OCH, or --OH and where R₂ = --H, --CH₃ or --C₂ H₅ and

where X = or C and

where n 1, 2 or 3

An example of this class of compounds is:

(a) 2-N-morpholinoethyl-1-naphthylamine

The chromogenic reagent is prepared by dissolving an appropriate amountof the chromogenic compound to be used in a solution containingapproximately 4 mol/L of sulfuric acid, and a surfactant such aspolyoxyethylene lauryl ether (such as BRIJ 35) or an alkylaryl polyether(such as TRITON) or any other non-ionic surface active agent in a finalconcentration of approximately 1-3%. The exact amount of chromogeniccompound to be used will be established by the molarity of theo-phthalaldehyde reagent used in the particular application. The generalguide lines are such that the molarity of the chromogenic compoundshould be ideally from about 0.1 to 1.0 times the concentration of theo-phthalaldehyde in the final reaction mixture. In general, the highermolar ratios of chromogenic compound to aldehyde may be used when thechromogenic compound does not possess a free amino group.

It will be understood that certain modifications and variations of thespecific and general concepts of the invention may be effected withoutdeparting from the inventive concepts heretofore described.

Accordingly, the invention is not to be considered limited to thespecific form or embodiments set forth herein for the purpose ofdisclosing and illustrating the inventive concepts discovered and hereinapplied.

For example, the preparation of the o-phthalaldehyde reagent isdescribed as being prepared in a solution of 3.75 N sulfuric acid. Thisparticular concentration of sulfuric acid has been found to provide anoptimal balance between a desirable reaction rate and the use of astrong acid. One practiced in the art, however, may deviate from the useof the exact concentration of acid given without departing from thegeneral concepts given herein. Similarly, one may substitute aparticular non-ionic surfactant such as described in any standard textand yet not depart from the basic inventive concepts detailed.

In the embodiments given above, the chromogen compound has been made upin an 8 N sulfuric acid solution. However, one may deviate from theexact normality given in the above preferred embodiment withoutdeparting from the inventive concepts described. The general guide linefor acid concentration departures is that either increasing ordecreasing the acid concentration of this reagent will lead to areduction in the rate of color development observed in the finalreaction mixture. While considerable variation in acid concentration maybe tolerated, sizeable decreases in acid concentration may lead to asignificantly diminished rate of color development; the use of sizeableincreases in acid strength may lead to diminished stability of reagentand the undesirable effects of exposing laboratory personnel andequipment to strong acids.

In the general guide lines for the preparation of the above reagents, itwill be noted that both desirably include the use of a non-ionic surfaceactive agent. The purpose of this surfactant may be twofold, dependingupon the particular chromogenic substance used.

For example, the inclusion of a surface active agent will generallyimpart better flow characteristics to the reagent system, thereforeproviding a more acceptable reagent for those analysts that will maketheir absorbance readings in a photometer equipped with a flow cell(i.e., a system which uses a single cell to measure all absorbances withan automatic means of filling and emptying the contents of the cell).

The second function of the surfactant used is to facilitate thesolubilization of the particular chromogenic compound used. While intheory all of the general classes of substances outlined will work, ithas been discovered that the inclusion of a proper concentration of asurfactant may be necessary to aid in the solubilization of theparticular compound used to obtain optimal results. For example whenclass 1 or class 2 compounds are used, it is necessary to include aproper concentration of surfactant to effect solubility and to preventturbidity in the final reaction mixture.

VI. EXAMPLES EXAMPLE 1 PREPARATION OF REAGENT SYSTEM

An o-phthalaldehyde reagent was prepared by dissolving 2000 mg ofo-phthalaldehyde in approximately 800 ml of 3.75 N sulfuric acid whichcontained 1 ml of a 25% solution of polyoxyethylene lauryl ether (BRIJ35) and the reagent made to volume of 1 liter with 3.75 N sulfuric acid.The receptor reagent was prepared by dissolving 500 mg of8-(4-amino-1-methylbutylamino)-6-methoxyquinoline phosphate in 800 ml ofa solution containing 5.0 gm of boric acid, 222 ml of concentratedsulfuric acid, and 0.9 ml of a 25% solution of polyoxyethylene laurylether (BRIJ 35), and the resulting solution made to a final volume ofone liter by addition of water.

ANALYTICAL PROCEDURE

The reagent system prepared as described above leads to a very rapidcolor development that is well suited to automated analysis.

In using this reagent system, approximately 30 microliters of a sampleof body fluid are added to a reaction tube; and to it simultaneously areadded approximately 1.8 ml of each of the above described reagents, andthe resultant mixture is thoroughly mixed to insure homogeneity. Thereaction mixture is then incubated at 37° C. to further hasten the colordevelopment for a period of three minutes. The reaction mixture is thentransferred to a spectrophotometer, and the absorbance of the resultingcolor is measured at a wavelength of 520 nm. The urea content of thepatient's sample is then automatically calculated from the absorbancereading of a standard solution of urea which has been treated in anidentical manner.

This particular reagent system and application has been shown to givelinear results with serum samples containing as much as 150mg/d1 of ureanitrogen, and to be virtually unaffected by sulfa drugs commonly used inthe treatment of kidney dysfunctions.

EXAMPLE 2 PREPARATION OF REAGENTS

The o-phthalaldehyde reagent is prepared by dissolving approximately 2grams of o-phthalaldehyde in one liter of 3.5 N sulfuric acid whichcontains 4 ml/L of TRITON X-100, and 1 ml/L of polyoxyethylene laurylether, (BRIJ 35) both of which are surfactants.

The receptor reagent is prepared by dissolving 1.8 gm of1,3-dihydroxynaphthalene in one liter of 5N sulfuric acid containing 15ml/L of TRITON X-100.

PROCEDURE

In this example, 20 microliters (0.02 ml) of a body fluid containing anunknown amount of urea is added to a tube containing 3.0 ml of thealdehyde reagent and mixed. One (1.0) ml of the receptor reagent,1,3-dihydroxynaphthalene, is added and mixed. The resultant mixture isthen incubated at 37° C. for 10 minutes, and the absorbance developed isread in a spectrophotometer versus a reagent blank composed of 20microliters of water, 3.0 ml of the aldehyde reagent and 1.0 ml of the1,3-dihydroxynaphthalene reagent which has been treated in a similarmanner, at a wavelength of 470 nm.

The absorbance of the unknown is then compared with the absorbancedeveloped in a standard solution of urea nitrogen which has been treatedidentically to the unknown for purposes of calculating the urea contentof the unknown.

EXAMPLE 3

All reagents are prepared as in Example 1 except that 600mg ofN-morpholinoethyl-1-naphthylamine is substituted for the 500 mg of8-(4-amino-1-methylbutylamino)-6-methoxyquinoline phosphate used in thechromogen reagent. This reagent system produces characteristics similarto those in Example 1 except that the final reaction product is read ata wavelength of 540 nm instead of 520 nm.

EXAMPLE 4

The reagents are prepared as in Example 2 with the exception that 1,880mg of 1,3,5-trihydroxybenzene is substituted for the 1.8 gm ofdihydroxynaphthalene used in the chromogen reagent. The performancecharacteristics and application of this system are essentially similarto those given in Example 2.

EXAMPLE 5

The reagents as prepared in Example 2 above with the exception that 291mg of 4,6-dihydroxy-2-aminopyrimidine is substituted for the 1.8 gm ofdihydroxynaphthalene used in the chromogen reagent. The performancecharacteristics and application of this system are essentially the sameas those given in Example 2 above.

REFERENCES

1. marshall, E. K., Jr.: J. Biol. Chem. 15:487 (1913)

2. Gentzkow, C. J.: J. Biol. Chem. 143:531 (1942)

3. Henry, R. J.: Clinical Chemistry: Principles and Techniques, NewYork, Harper & Row (1968) p. 513

4. Fearon, W. R.: Biochem J.: 33:902 (1939)

5. ormsby, A. A.: J. Biol. Chem.; 146:595 (1942)

6. Brown, H. H., Anal. Chem. 31:1844 (1959)

7. Roijers, A. F. M. and Tas, M. M., Clin. Chem. Acta, 9:197 (1964)

8. Morin, L. G., and Prox, J., Clin. Chem. Acta, 47:27 (1973)

9. Jung et al. Clin. Chem., 21:1136 (1975)

10. Merck Index, 8th Ed., p. 717 Merck and Co., Inc. (1968)

I claim:
 1. A process for the quantitation of urea in a fluid sample,comprising reacting the sample with a reagent containing an acidicsolution of o-phthaladehyde and an acidic solution of a chromogeniccompound of the general class of compounds which have the followinggeneral structure: ##STR7## where R₁ = -H, --OCH, or --OH and where R₂ =-H, --CH₃ or --C₂ H₅ andwhere X = O or C and where n = 1, 2 or 3 andmeasuring the absorbance of the reacted sample mixture.
 2. The processas defined in claim 1, in which the chromogenic compound used is2-N-morpholinoethyl-1-naphthylamine.